Coal is one of the most abundant sources of fuel known. However, the quality and efficacy of different coals ranges widely, depending on where the coal is mined and the uses to which it is to be put. Coal generally contains moisture in amounts of up to about 50% by weight, which adds to coal transportation costs, decreases the heat value of the coal and favors formation of acid rain precursors upon burning the coal.
Generally, in order to burn efficiently, it is first necessary for the hydrocarbon components of coal to absorb heat, in order to liberate the moisture present and cause a molecular transformation of the complex hydrocarbons contained in the coal into more simple, more readily combustible hydrocarbons. This heat absorption is generally accomplished in the combustion zones of boilers and furnaces into which the coal is fed. However, this is a highly inefficient way to process the coal fuel, particularly for the lower rank, high moisture content coals and lignites, which require considerable energy and time for drying and for molecular transformation. Requiring the coal to absorb heat in the combustion zone also contributes to the production of both NO.sub.x and SO.sub.2, the precursors of acid rain, since considerable excess air at elevated temperature and pressure is required to maintain suspension for the extended time required to burn the coal. This in turn provides excess oxygen for reaction with the sulfur and nitrogen in the combustion zone and in the flue gas stream.
The prior art contains numerous attempts to solve some or all of the above shortcomings of coal. Buck, U.S. Pat. No. 1,925,132 discloses a process of pretreating coal to 250.degree.-450.degree. F. to reduce moisture content and improve burning efficiency. However, this method only reduces moisture levels down to about 7% by weight, which precludes providing the heat energy necessary to simplify the molecular structure of the coal.
Other prior art techniques utilize high temperatures to drive off the moisture from the coal. See, for example, Wingett, U.S. Pat. No. 1,337,496. However, such high temperatures (800.degree. C.) tend to drive off volatile components in the coal as well, thereby lessening its fuel value, and further tend to cause the coal to become exothermic.
Accordingly, it would be useful to provide a method of treating coal to solve some or all of the above-noted problems.
It is therefore an object of the invention to provide a method for increasing the rank of coal.
It is another object of the invention to lower the ignition temperature of certain treated coals relative to untreated (raw) coal.
It is another object of the invention to provide a method for treating coal to reduce the formation of acid rain precursors.
It is still another object of the invention to provide a method of treating coal and thereby remove substantially all of the moisture from the coal.
It is a further object of the invention to render the treated coal substantially impenetrable to moisture reabsorption.
These and other objects of the invention will become apparent as the following detailed description of the preferred embodiments of the invention proceeds.